This invention relates to a method of fabricating a magnetic bubble memory device in which the patterned layer of magnetically soft material defining bubble propagation path elements and bubble function-determining components is disposed in a planar configuration, and to magnetic bubble memory structures resulting therefrom. More particularly, the fabrication method in accordance with this invention involves the consecutive deposition onto a substrate having a bubble-supporting magnetic film, of a plurality of layers of differing materials including a layer of non-magnetic electrically conductive material, a layer of insulating material, and a layer of magnetically soft material, and thereafter patterning each of the respective layers in a top-down sequence beginning with the uppermost layer and continuing downwardly in forming an upper patterned metal layer of magnetically soft material defining elements of the magnetic bubble propagation paths and bubble function-determining components, insulation spacers, and a lower patterned metal layer of non-magnetic electrically conductive material defining control conductors. In the completed magnetic bubble memory device, the patterned planar layer of magnetically soft material is uniformly spaced above the bubble-supporting magnetic film, thereby contributing to uniformity in the magnetic field strengths induced under each of the magnetically soft elements defined in the patterned layer of magnetically soft material.
A magnetic bubble memory device comprises a substrate of non-magnetic material on which a planar film or layer of magnetic material capable of supporting magnetic bubbles is disposed. The magnetic bubbles are caused to travel along predetermined paths within the layer of bubble-supporting magnetic material by laying down a magnetizable bubble propagation path pattern above the layer of magnetic material as a series of thin film propagation elements of magnetically soft material, e.g. permalloy, in the form of tiny geometric shapes or circuit elements. A magnetic drive field is provided within the plane of the layer of magnetic material and is rotated to cause the individual propagation elements included in the bubble propagation path pattern to be sequentially polarized in a cyclical sequence causing the individual bubbles to be propagated in a step-wise movement along the path as defined by the magnetizable propagation elements. One such overlay pattern commonly employed in a magnetic bubble memory device is the so-called asymmetric chevron pattern wherein individual propagation elements may take the form of asymmetric chevron permalloy structures. Another such overlay pattern is the so-called series of alternating T-shaped and bar-shaped permalloy elements. The overlay pattern of magnetically soft material is typically organized into a storage section comprising one or more memory storage loops, each of which has a plurality of bit positions for accommodating magnetic single-walled domains or bubbles which respectively represent one bit of binary information. Thus, information in the form of a series of magnetic bubbles and voids respectively representing binary "1' s" and "0's" may be disposed in the respective memory storage loops for rotation thereabout in a synchronized and controlled manner to enable access to the stored information imparted thereby to be obtained. The memory storage section may be organized as a plurality of minor storage loops associated with a major storage loop, wherein the data information represented by magnetic bubbles and voids is transferred between the major loop and each of the respective minor loops, thereby enabling information to be read from the memory and to be written into the memory. In another form of magnetic bubble memory device architecture, the memory storage loops may be disposed between input and output sections such that information may be written into the storage loops via transfer gates interposed between the respective storage loops at one end thereof and a propagation path included in the input section, while information may be read out from the storage loops via output replicate gates to a propagation path included in the output section to provide non-destructive readout of data by replicating respective bubbles in the storage loops as these bubbles as being directed onto the propagation path in the output section for subsequent sensing by a bubble detector and erasure by an annihilator.
Heretofore, typical fabrication practices in constructing a magnetic bubble memory device have produced offset portions or "steps" in the overlay pattern of magnetically soft material which includes individual bubble propagation path elements defining the bubble propagation paths within the bubble-supporting magnetic film therebeneath and the bubble function-determining components, such as generators, replicators, annihilators and transfer gates, for example. This offset configuration occurs in areas of the overlay pattern of magnetically soft material which are disposed above a lower metallization level comprising an overlay pattern of non-magnetic electrically conductive material disposed above the bubble-supporting magnetic film but separated from the overlay pattern of magnetically soft material by insulating material. In this respect, the first or lower overlay pattern of non-magnetic electrically conductive material defines control conductors which extend beneath bubble function-determining components, such as generators, replicators, annihilators and transfer gates. The conventional fabrication technique provides for laying down insulating material, such as silicon dioxide, to cover the first level metallization comprising the overlay pattern of non-magnetic electrically conductive material, followed by the deposition of the second or upper metallization layer comprising the layer of magnetically soft material which is subsequently patterned. The presence of the lower overlay pattern of non-magnetic electrically conductive material and the covering body of insulating material causes the deposition of the layer of magnetically soft material to assume a non-planar configuration including offset portions in each of the areas overlying control conductors defined by the lower overlay pattern of non-magnetic electrically conductive material.
In order to increase data bit density per unit area in a magnetic bubble memory chip, it would be desirable to reduce the size of the magnetic bubbles and the bubble circuit period. In this connection, magnetic bubble memory chips have been operated with magnetic bubbles of five micron size, where the bubble function-determining components as defined by the upper non-planar overlay pattern of magnetically soft material have operated in a reliable manner in propagating bubbles in guided paths about the bubble-supporting magnetic film of the memory chip. In this instance, the transfer gates between the bubble storage section and a major propagation path to exchange data in the form of chains of magnetic bubbles and voids and the output replicate gates between the bubble storage section and a major propagation path for non-destructive readout of data, although being of non-planar configuration by virtue of the fabrication method employed in constructing the magnetic bubble memory chip have performed in a satisfactory manner. However, continuing efforts to reduce the bubble circuit period, as from the 12-16 .mu.m range typically employed to the 6-8 .mu.m period range enabling the use of bubbles having a size of 2 microns have encountered problems insofar as the reliable operation of the transfer gates and output replicate gates are concerned because of the heightened effect of bubble propagation anomalies induced by the step coverage of the magnetically soft material over the non-magnetic, electrically conductive control conductors brought about by the shrinkage in the bubble circuit period. With such reduced bubble circuit periods in a magnetic bubble memory device fabricated by a conventional non-planar process, transfer gate regions and output replicate gate regions as defined by non-planar portions of permalloy have produced sporadic results introducing serious reliability factors into the operation of such magnetic bubble memory devices.
To this end, a number of so-called planar fabrication processes have been developed to eliminate the non-planar character of the gate regions as defined by the upper overlay of magnetically soft material. These planar processes generally involve piece-meal processing from the lower metallization level upwardly, wherein the non-magnetic electrically conductive overlay pattern is first accomplished to define the control conductors and the respective leads, subsequently followed by the deposition and patterning of the upper metallization level of magnetically soft material over a body of insulation material covering the lower patterned metallization level and filling the voids therein. Such so-called planar processes suffer from the disadvantage of restrictions being placed on the geometry reductions which might be accomplished, since a substantial portion of the patterning complexity exists in the upper metallization layer of magnetically soft material. In an effort to attack this small geometry resolution problem, a top-down planar process has been previously suggested in which a sandwich structure of individual planar layers including a first layer of insulation material, a layer of non-magnetic electrically conductive material, a second layer of insulation material, and a layer of magnetically soft material is disposed on the bubble-supporting magnetic film, followed by patterning of the upper planar layer of magnetically soft material and subsequent patterning of the second layer of insulation material and the layer of non-magnetic electrically conductive material. This top-down planar process attempted to employ a double density photoresist mask so as to provide different thicknesses of photoresist over the permalloy and control conductor elements. Although control conductor metal is produced by such a process beneath the permalloy elements defined in the planar overlay pattern forming the upper metallization layer, efforts to develop a magnetic bubble memory device which could operate successfully when fabricated in this manner have heretofore produced inconsistent and generally unsatisfactory results.